1. Field of the Invention
The present invention relates to a transmission power control method and apparatus for a mobile radio satellite communication system, and more particularly to a transmission power control method and apparatus for a mobile radio satellite communication system which controls the transmission power based on received control information of a transmission power particularly when a channel is switched.
2. Description of the Related Art
A satellite communication system which effects channel access of the single channel per carrier (SCPC)/frequency division multiple access (FDMA) system based on a demand assignment is described first. Referring to FIG. 3, a satellite communication system of the type mentioned is shown and normally includes a network control station 11 which manages and controls the communications system, gateway stations 12 each connected to public switched telephoney networks (PSTN) or private telephone networks, terminals and mobile stations (hereinafter referred to as terminals/mobile stations) 13 through which users access the satellite communication network, and a communication satellite 14.
The network control station 11 supervises the entire network and successively transmits channel signalling information by time division multiplex (TDM) communication using a forward link (also called outbound). The TDM communication includes information of an incoming call and a used communication channel. Each of the gateway stations 12 and the terminals/mobile stations 13 receives information transmitted thereto by the time division multiplex communication and effects call request and responding to an incoming call based on the received information using a return link (inbound). The return link is used for transmission of a call request and incoming call responding burst signal transmitted from each of the gateway stations 12 and terminals/mobile stations 13 in a predetermined time slot in accordance with line information of time division multiplex communication in a time division multiple access (TDMA).
If the network control station 11 receives a call request and an incoming call response signal transmitted from one of the gateway stations 12 and one of the terminals/mobile stations 13 (including an indirect case wherein a terminal or a mobile station transmits a call request to a gateway station and then the gateway station transmits a channel assignment request), then it informs the gateway station 12 and the terminal/mobile station 13 of an available communication channel over a signalling channel 15. Then, when the gateway station 12 and the terminal/mobile station 13 receive the designated communication channel, they set the channel so that a communication channel 16 is thereafter used in place of the signalling channel 15. Then, through a predetermined sequence, the terminal/mobile station 13 is connected to a PSTN or private switched network (PSTN)/private switched network 17) through the communication satellite 14 and the gateway station 12.
The signalling channel 15 which communication control information therethrough in such a sequence of channel connection operations as described above communicates control information normally using a prescribed maximum transmission level as a standard level so that call connection can be effected stably even in a geographical condition wherein the elevation angle is small (which is the most critical communication condition in call connection) in order to assure a high degree of reliability of the communication system. After changeover to a communication channel, the transmission level is adjusted so as to minimize interference with another communication channel and allow the most efficient utilization of power of the satellite. Further, in a mobile terminal which is driven by a battery, power dissipation is minimized by such transmission level control to allow utilization for a long time.
The transmission powers of the gateway station 12 and the terminal/mobile station 13 or the communication system described above are controlled each by such an apparatus as shown in FIG. 4. Referring to FIG. 4, the apparatus shown includes a transmission power controller 1 for controlling the transmission power of a modulation signal frequency converted by a frequency converter not shown, a power amplifier 2 for power amplifying an output of the transmission power controller 1 to a predetermined transmission power and outputting the power amplified signal to an antenna not shown, a level detector 3 for detecting the output level of the power amplifier 2, a comparator 6 for comparing the transmission level detected by the level detector 3 with a preset transmission reference level to obtain a level error, and an averaging circuit 9 for averaging the level error from the comparator 6 and controlling the transmission power controller 1 with the averaged level error.
In operation, upon transmission over the signalling channel 15, the transmission level is set to the maximum standard transmission level, and the level of a transmission signal detected by the level detector 3 during transmission is compared with the preset transmission level value by the comparator 6 to obtain a level error. The level error of the detected level from the preset transmission reference level is averaged by the averaging circuit 9. Then, the transmission power is controlled so that, if the detected transmission signal level is higher than the preset transmission level value, then the transmission signal level may be decreased, but if the detected transmission signal level is lower than the preset transmission level, then the decrease of the transmission signal level may be decreased.
A channel is assigned in response to the channel assignment request in the signalling channel 15, and communication by the communication channel 16 is started. After the communication by the communication channel 16 is started, the terminal/mobile station 13 and the gateway station 12 individually measure the reception levels thereof, set optimum transmission levels for operation of the system and control the transmission powers thereof. Such control is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 8-237194. The transmission power control for the communication channel 16 is performed in a similar manner as that for the communication channel 16 except that the preset transmission level values set for the comparators are different.
After the communication over the communication channel 16 is completed, communication by the signalling channel 15 is entered again. Then, the transmission signal level is set to the standard maximum transmission level, but is then controlled, upon transmission, to the predetermined transmission power by the operation described above.
While the transmission level is controlled in accordance with the procedure described above so that possible interference between communication channels may be minimized and the power of the satellite may be utilized most efficiently, if the control step of the transmission level becomes large, the following trouble occurs.
FIG. 5 represents transmission powers when a channel to be used is changed over between a signalling channel and a communication channel.
When the transmission level of the self station after communication over the communication channel is started is to be decreased to the lowest level because the reception level of the other party station is sufficiently high, it is varied at a rate which can be followed up sufficiently so that synchronization can be held on the reception side against a variation in amplitude or phase. When the communication is terminated while the transmission power is controlled stably with the lowest transmission level and then communication over the signalling channel is entered again, the transmission level is set to the standard highest transmission level in preparation for next transmission. When a control signal burst such as a call request or an incoming call response is to be transmitted subsequently over the signalling channel, the top of the burst is transmitted with the level controlled to the lowest level over the communication channel and the burst is started while the transmission level is controlled so as to increase to the highest level.
Therefore, it sometimes occurs that a carrier power to noise power ratio required by the communication system is not obtained and this gives rise to a trouble in synchronization acquisition or data transmission. Particularly, a top portion of a burst includes a training sequence and/or a unique word for carrier synchronization and/or clock synchronization and plays an important role for transmission of a data part.
If, on the contrary, the transmission level control response over the signalling channel is made faster so as to allow quick convergence to a predetermined transmission level in order to eliminate the problem described above, then such a problem may possibly occur that, when the transmission level is controlled after reception level information of the other party station is received over the communication channel, frame synchronization of the other party station cannot be maintained or that the transmission power control becomes unstable. Such problems are significant particularly where the transmission level control step is large.